Important Clue Uncovered for the Origins of a Type of Supernovae Explosion,
Thanks to a Research Team at the University of Pittsburgh

Pitt team used Sloan Digital Sky Survey to determine that the merger
of double white dwarfs is a plausible explanation for Type Ia supernovae

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This series of artist's concepts shows two white dwarfs orbiting one another. In the future,
their orbits will get smaller and smaller,
and faster and faster, until someday they merge and explode.
Image Credit: NASA/GSFC/D.Berry

PITTSBURGH — An important clue has been uncovered about the origins of an important type of exploding star,
Type Ia supernovae, thanks to a research team at the University of Pittsburgh. Studying supernovae of this type helps researchers
measure galaxy distances and can lead to important astronomical discoveries. A paper detailing this research has been accepted for
publication in Astrophysical Journal Letters.

Principal investigator Carlos Badenes, assistant professor of physics and astronomy in Pitt’s Kenneth P. Dietrich School of Arts and
Sciences, detailed the ways in which his team used the Sloan Digital Sky Survey—a collection of multicolor images and more than a
million spectra covering more than a quarter of the sky—to determine what kinds of starts produce Type Ia supernovae explosions.

“We knew that two stars had to be involved in such an explosion, and that one of them had to be a white dwarf,” says Dan Maoz,
professor of physics and astronomy at Tel-Aviv University in Israel and coauthor of this soon-to-be-published paper on the discovery.
“But there were two possibilities for what the second star is, which is what we sought to discover.”

According to Badenes, there were two potential outcomes for the star’s type. It could be a “normal star,” like the sun, or it could
be another white dwarf, which is a smaller, more dense faint star composed of electron-degenerate matter.
The team suspected the latter, as two white dwarfs within the same star system would revolve around one another at half a million miles
an hour, speeding up and getting closer and closer until one day they merge, most likely producing the fireworks of Type Ia supernovae.

“There were obvious reasons to suspect that Type Ia supernovae come from the merging of a double white dwarf,” says Maoz.
“But our biggest question was whether there were enough double white dwarfs out there to produce
the number of supernovae that we see.”

Because white dwarfs are extremely small and faint, there is no hope of seeing them in distant galaxies. Therefore,
Badenes and Maoz turned to the only place where they could be seen: the part of the Milky Way Galaxy within about a thousand light
years of the sun. To find the star’s companion, the team needed two spectra to measure the velocity between the two. However,
SDSS only took one spectrum of most objects. The team decided to make use of a little-known feature in the SDSS spectra to separate
each one into three or more subspectra. Although the reprocessing of the data was challenging, said Badenes, the team was able to
compile a list of more than 4,000 white dwarfs within a year, each of which had two or more high-quality subspectra.

“We found 15 double white dwarfs in the local neighborhood and then used computer simulations to calculate the rate at which double
white dwarfs would merge,” says Badenes. “We then compared the number of merging white dwarfs here to the number of Type Ia supernovae
seen in distant galaxies that resemble the Milky Way.”

This image mosaic shows 99 of the nearly 4,000 white dwarfs that Badenes and his colleagues examined. Of the four thousand,
they found fifteen double white dwarfs.
Image Credit: Carles Badenes and the SDSS-III team

The result was that, on average, one double white dwarf merger event occurs in the Milky Way about once a century.

“That number is remarkably close to the rate of Type Ia supernovae we observe in galaxies like our own,” says Badenes.
“This suggests that the merger of a double white dwarf system is a plausible explanation for Type Ia supernovae.”

In addition to providing a key clue about the nature of these important events, the team’s discovery shows the potential of giant
astronomical surveys like the SDSS.

“Twenty years ago we decided to take three subspectra for each spectrum. We did that for entirely practical reasons,” says
Robert Lupton, senior research astronomer in Princeton University’s Department of Astrophysical Sciences and a colleague of Badenes.
“We had no idea that it would someday give us an important clue to the mystery of the Type Ia supernovae.”